Add documentation and small script to test external deep links.

tools/adb/deeplink_external.sh

@@ -0,0 +1,19 @@
+#! /bin/bash
+#
+# Copyright (c) 2024 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+adb shell am start -a android.intent.action.VIEW -c android.intent.category.BROWSABLE \
+   -d "https://app.element.io/#/room/!cuqHozLHNBgupgLMKN:matrix.org/%24LZDOueY3R8OD2ZYf8FLKtu95aF7imLBC3F5TIUj-4cc"